Ok I feel stupid I know I've found it before but I'm having a major brain fart where in the xen base settings is the new post indicator?
It's a default XenForo property: Style Properties: Message Elements, is that what you're looking for? New Indicator Inner/Outer
Just like these here new post indicators here

Style Properties: ├ ─ Nodes, the option to enable / disable is there, then on the left side: Node New Indicator CSS to style it / change the text.

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